I think your Vinca has a fungal disease. There are two types of fungus that Vinca acquires usually in the early Spring when it is damp and cool. If you fertilize at that time it will make it worse. Sunnier, drier locations are better to retard the diseases.

Both diseases have the exact same symptoms--browning of the foliage and then dying. Without a biochem lab it is usually not possible to tell which.

I had this very problem and it was solved once and for all by a fungal treatment. Here is what I do. In March as long as the ground is not frozen, I spray the foliage and soil drench with Cleary's 3336 (Thiophanate Methyl), and repeat every 30 days until early June. If I still see wilting, I know it is other disease and treat the bed in the same way with Banner Maxx.

This season my Vinca is coming up beautiful, strong, blooming, vigorous, and we have had a very cold, wet Spring.